The mg 42 is an awesome build, however receivers are very expensive and hard to get now. you can get an mg34 kit and receiver semi auto kit from TNW for about 1,600. 00 mg34 cut receivers are also very hard to get. both builds are basically TIG welding projects.
You can get a bely fed RPK from vector for 2K, I have one and they are awesome
You can get a TNW semi mg34 for about $3800 TNW also is working on a semi auto mg42.
Hope that helps.